Space between a drop ceiling in a building and a structural floor from which it is suspended serves as a return-air plenum for elements of heating and cooling systems as well as a convenient location for the installation of communications cables. The latter includes voice communications, data and signal cables for use in telephone, computer, control, alarm and related systems. It is not uncommon for these plenums to be continuous throughout the length and width of each floor.
When a fire occurs in an area between a floor and a drop ceiling, it may be contained by walls and other building elements which enclose that area. However, if and when the fire reaches the plenum, and if flammable material occupies the plenum, the fire can spread quickly throughout an entire floor of the building. The fire could travel along the length of cables which are installed in the plenum if the cables are not rated for plenum use. Also, smoke can be conveyed through the plenum to adjacent areas and to other floors.
As the temperature in a non-plenum rated cable rises, charring of the jacket material begins. Afterwards, conductor insulation inside the jacket begins to decompose and char. If the jacket char retains its integrity, it functions to insulate the core; if not, it ruptures due to either expanding insulation char or pressure of gases generated from the insulation, exposing the virgin interior of the jacket and insulation to elevated temperatures. The jacket and the insulation begin to pyrolyze and emit more flammable gases. These gases ignite and, because of air drafts within the plenum, burn beyond the area of flame impingement, propagating flame, and generating smoke and toxic and corrosive gases.
As a general rule, the National Electrical Code (NEC) requires that power-limited cables in plenums be enclosed in metal conduits. However, the NEC permits certain exceptions to this requirement provided that such cables are tested and approved by an independent testing agent, such as Underwriters Laboratories (UL), as having suitably low flame spread and smoke-producing characteristics. The flame spread and smoke production of cable are measured using UL 910, Standard Test Method for Fire and Smoke characteristics of Electrical and Optical-Fiber Cables Used in Air-Handling Spaces. See S. Kaufman "The 1987 National Electric Code Requirements for Cable" which appeared in the 1986 International Wire and Cable Symposium Proceedings beginning at page 545.
Commercially available fluorine-containing polymer materials have been accepted as the primary insulative covering for conductors and as a jacketing material for plenum cable without the use of metal conduit. However, fluoropolymer materials generate corrosive gases under combustion conditions. Also, some fluorine-containing materials have a relatively high dielectric constant which makes them unacceptable as insulation for communications conductors.
The problem of acceptable plenum cable design is complicated somewhat by a trend to the extension of the use of optical fiber transmission media from a loop to building distribution systems. Not only must the optical fiber be protected from transmission degradation, but also it has properties which differ significantly from those of copper conductors and hence requires special treatment. Light transmitting optical fibers are mechanically fragile, exhibiting low strain fracture under tensile loading and degraded light transmission when bent with a relatively low radius of curvature. The degradation in transmission which results from bending is known as microbending loss. This loss can occur because of coupling between the jacket and the core. Coupling may result because of shrinkage during cooling of the jacket and because of differential thermal contractions when the thermal properties of the jacket material differ significantly from those of the enclosed optical fibers.
The use of fluoropolymer materials for optical fiber plenum cable jackets requires special consideration of material properties such as crystallinity, and coupling between the jacket and an optical fiber core which can have detrimental effects on the optical fibers. If the jacket is coupled to the optical fiber core, the shrinkage of semi-crystalline, fluoropolymer plastic material, following extrusion, puts the optical fiber in compression and results in microbending losses in the fiber. Further, its thermal expansion coefficients relative to glass are large, thereby compromising the stability of optical performance over varying thermal operation conditions. The use of fluoropolymers also is costly and requires special care for processing.
Further, a fluoropolymer is a halogenated material. Although there exist cables which include halogen materials and which have passed the UL 910 test requirements, there has been a desire to overcome some problems which still exist with respect to the use of halogenated materials such as fluoropolymers and polyvinyl chloride (PVC). These materials, under combustion conditions, generate substantial levels of corrosive gases. Depending on the fluoropolymer used, hydrogen fluoride and hydrogen chloride can form under the influence of heat, causing corrosion and, according to some tests, increased toxicity. With PVC, only hydrogen chloride is formed.
Non-halogenated materials have been suggested for use as insulating and jacketing material for cables. See application Ser. No. 07/303,172 filed on Jan. 27, 1989 in the name of T. G. Hardin, et al. now U.S. Pat. No. 4,941,729 having issued on July 17, 1990.
A problem relating to the use of commercially available non-halogenated plastic materials has surfaced. Generally speaking, the non-halogenated materials which are available commerically are injection molding grade materials which are intended for uses in which the thickness of the molded material is substantially greater than the 5 to 15 mils that might be expected for use as conductor insulation. The melt index for the available non-halogenated materials is relatively low, being, for example in the range of 0.75 to 1.5. As is well known, melt index is indicative of the flow properties of a plastic material. The higher the melt index, the better the flow. Lower melt index materials require higher barrel extruder temperatures which could result in degradation of the plastic material. When plastic materials to be used for insulation, for example, degrade in the barrel of an extruder, acid rings, which have a propensity to cling to materials which they contact, are formed. As a result, physical properties of the insulation, such as its adhesion to an enclosed conductor, are unsatisfactory. Also, inconsistent original and post-aging physical properties are a consequence of degradation.
For jacketing, the same results have been observed. Quality controlled compositions result in better dispersion, but long term aging properties still are marginal.
What is needed is a cable which includes non-halogenated materials and which overcomes the hereinbefore discussed problems of the prior art. The sought-after cable not only exhibits suitably low flame spread and low smoke producing characteristics provided by currently used cables which include halogenated materials but also one which meets a broad range of desired properties including improved corrosivity. Such a cable does not appear to be available in the prior art. Quite succinctly, the challenge is to provide a halogen-free cable which meets the standards for plenum cables and which provides sought after properties such as suitable plastic-to-conductor adhesion and desirable physical properties which are retained post-processing. What is further sought is a cable which is characterized as having relatively low corrosion properties and acceptable toxicity, low levels of smoke generation, and one which is readily processable at reasonable cost.